1. Field of the Invention
The present invention relates to an electrically operated variable valve timing control system for an automobile engine which alters the timing at which valves are opened and closed in accordance with driving conditions.
2. Description of Related Art
Some electrically operated variable valve timing control devices of this kind change the timing at which engine valves are opened and closed by electrically operated components via the application and release or interruption of electric voltage. For example, Japanese Unexamined Patent Publication No. 63-162,910 describes a valve timing adjustment device which is equipped with an electrically operated retarder or valve timing adjustment device capable of applying and releasing pressure to, or energizing and de-energizing, a valve timing advancing ring for altering the valve timing between two different valve timings.
Such a valve timing adjustment device has an advancing component, such as an advancing control ring, movable within specified limits in an axial direction of a camshaft for driving valves in a timed manner. The control ring is operationally connected to a hub mounted to the camshaft and to a rotary input member, such as a sprocket, cooperating with an engine crankshaft. Axial movement of the advancing control ring causes a relative rotation between the hub and the rotary input member. An advancing ring and the hub, which are connected to the advancing control ring so as to rotate relative to each other, are threadingly coupled. The valve timing adjustment device further has a stationary or non-rotatable retarder, which may include an electro-magnetic coil, which is placed so as to face the advancing ring. When electric voltage is applied to the electro-magnetic retarder, the retarder is forced by the electro-magnetic force to press against the advancing ring, applying braking force onto the advancing ring, causing only a specified angle of relative rotation of the advancing ring. With this relative rotation of the advancing ring, the advancing control member moves in the axial direction of the cam shaft, so that the rotational phase of the cam shaft is altered or retarded with respect to the rotation of the rotary input member. In such a way, the timing at which intake valves or exhaust valves are opened and closed is altered between when electric voltage is applied to the electro-magnetic retarder and when electric voltage is released or withdrawn.
For altering a valve timing, it is typical to install a variable valve timing device so as to drive or open and close valves, for instance intake valves, at different valve timings. Such valve timings may include a standard valve timing, which is identical to a general valve timing of a fixed valve drive mechanism, and a specific valve timing, retarded a specific time with respect to the standard valve timing so as to cause a decrease in compression volume. The intake valves are opened and closed at the specific valve timing when the engine operates in a range of engine operating conditions in which an improvement in fuel consumption is especially demanded. When incorporating an electrically operated variable valve timing device, such as an electro-magnetic retarder, it is typical to withdraw electric voltage from the device in order to establish the normally required standard valve timing and to impress or apply voltage to the device so as to establish the specific valve timing which is less frequently required.
However, if the electrically operated variable valve timing device is thus structured, even if a pumping loss is reduced by operating the intake valves at the specific valve timing in a range of engine operating conditions in which improved fuel consumption is required, an improvement in fuel consumption is not always accomplished. This is due to the consumption of electric power for electrically operating the variable valve timing device. In addition, improvements must be made in enlarging a range in which pumping losses should be reduced, efficiently performing deceleration in a range in which deceleration is required, reducing torque shocks, etc.